Display and lighting systems based on LEDs (Light Emitting Diodes) have a variety of applications. Such display and lighting systems are designed by arranging a plurality of photo-electronic elements (“elements”) such as rows of individual LEDs. LEDs that are based upon semiconductor technology have traditionally used inorganic materials, but recently, the organic LED (“OLED”) has come into vogue. Examples of other elements/devices using organic materials include organic solar cells, organic transistors, organic detectors, and organic lasers.
An organic OLED is typically comprised of two or more thin organic layers (e.g., an electrically conducting organic layer and an emissive organic layer where the emissive organic layer emits light) which separate an anode and a cathode. Under an applied forward potential, the anode injects holes into the conducting layer, while the cathode injects electrons into the emissive layer. The injected holes and electrons each migrate (under the influence of an externally applied electric field) toward the oppositely charged electrode and produce an electroluminescent emission upon recombination in the emissive layer. Similar device structure and device operation applies for OLEDs consisting of small molecule organic layers and/or polymeric organic layers. Each of the OLEDs can be a pixel element in a passive/active matrix OLED display or an element in a general area light source and the like. The displays and light sources may have one or more common layers such as common substrates, anodes or cathodes and one or more active/passive organic layers sandwiched in between to emit light in particular spectra. They may also consist of photo-resist or electrical separators, bus lines, charge transport and/or charge injection layers, and the like. Typically, a transparent or semi-transparent glass substrate is used in bottom-emitting OLED devices.
Organic light-emitting diodes (OLEDs) based on small molecules or polymers are being considered as a very promising potential technology for solid state lighting applications. OLED light sources can be used for many special or general illuminating applications, for example, it can be used as a light source for the treatment of human diseases. Compared to inorganic LEDs, OLEDs have shown many special advantages, such as easy fabrication, high throughput, large emissive area, and so on. For lighting applications, high device efficiency is desired for low power consumption, low device temperature rise during operation, and other beneficial effects. For OLEDs, the biggest limiting factor for its efficiency is the wave guiding effect within the device and substrate. As a result, only less than 40% of the totally generated light in the device can escape the front glass and illuminate the needed area. Therefore there is a need to design efficient OLED light sources for many applications.